1. Field of the Invention
The present invention relates to a photoelectric conversion device, a method of manufacturing the device, and an X-ray imaging system including the device. More particularly, it relates to a photoelectric conversion device wherein a plurality of sensor cells, in each of which a photoelectric element and a switching element are connected, are arrayed in two dimensions on a substrate, a method of manufacturing the device, and an X-ray imaging system including the device.
Also, the present invention is well suited for applications to a photoelectric conversion device which has photoelectric elements arrayed in two dimensions for actual-size reading in, for example, a facsimile equipment, a digital copier or an X-ray imaging apparatus, and to a method of repairing the photoelectric conversion device.
Further, the present invention pertains to an X-ray imaging system in which the above photoelectric conversion device having the photoelectric elements of two-dimensional arrayal is assembled.
2. Related Background Art
Heretofore, a reading system which is configured of a scaling-down optical system and a CCD (charge-coupled device) type sensor has been employed as the reading system of a facsimile equipment, a digital copier, an X-ray imaging apparatus or the like. In recent years, however, photoelectric semiconductor materials typified by hydrogenated amorphous silicon (hereinbelow, expressed as "a-Si") have been being developed. This has resulted in the remarkable developments of so-called "close contact type sensors" in each of which photoelectric elements and a signal processing portion are formed on a substrate of large area so as to adopt an optical system for the actual-size reading of an information source.
In particular, the a-Si is usable, not only as the photoelectric material, but also as the material of thin-film field effect transistors (hereinbelow, simply expressed as "transistors"). Accordingly, it has the advantage that a semiconductor layer for photoelectric conversion and a semiconductor layer for the transistors can be formed at the same time.
An example of a photoelectric conversion device utilizing such a-Si is disclosed in the official gazette of Japanese Patent Application Laid-open No. 8-116044. It will now be explained with reference to FIG. 1 and FIGS. 2A and 2B of the accompanying drawings. FIG. 1 is a circuit diagram showing the whole circuit of the photoelectric conversion device. Besides, FIG. 2A is a schematic plan view of constituent elements which correspond to one sensor cell of the photoelectric conversion device. Further, FIG. 2B is a schematic sectional view taken along line 2B--2B indicated in FIG. 2A.
First, the construction of the photoelectric conversion device will be explained. Referring to FIG. 1, each sensor cell is configured of a photoelectric element S, a capacitor C and a transistor T. In the photoelectric conversion device, the sensor cells totaling nine (3.times.3) are divided into three blocks of respective columns. That is, one block consists of three sensor cells.
In the figure, symbols S11 to S33 denote the photoelectric elements S. The lower electrode side of each photoelectric element S is indicated by letter G, while the upper electrode side thereof is indicated by letter D. In addition, symbols C11 to C33 denote the capacitors for storage, and symbols T11 to T33 the transistors for transferring data.
Besides, symbol Vs designates a power source (or supply voltage) for reading out a converted charge signal, and symbol Vg a power source (or supply voltage) for refreshment. These power sources Vs and Vg are respectively connected to the G electrodes of all the photoelectric elements S11 to S33 through a switch SWs and a switch SWg. The switch SWs is connected to a refreshment control circuit RF through an inverter, while the switch SWg is connected thereto directly. The switch SWg is controlled so as to turn "ON" during a refreshing time period.
Further, a part enclosed with a broken line in FIG. 1 is formed on an identical insulated substrate of large area. In the enclosed part, the sensor cell having the photoelectric element S11 is illustrated as the plan view in FIG. 2A. Also, a plane along the dot-and-dash line 2B--2B indicated in FIG. 2A is illustrated as the sectional view in FIG. 2B.
Referring to FIG. 2B, the sensor cell generally shown in FIG. 2A includes a lower electrode 1 which forms a gate electrode on the insulating substrate, a gate insulator film 2, an i-layer 3 which is a semiconductor layer effecting photoelectric conversion, an n-layer 4 which hinders the injection of holes, and an upper electrode layer 5 which forms source and drain electrodes. This sensor cell is fabricated in such a way that the lower electrode layer 1, the gate insulator film 2, the i-layer 3, the n-layer 4, and the upper electrode layer 5 serving as the source and drain electrodes are first stacked in the order mentioned, that the upper electrode layer 5 is subsequently etched to form the source and drain electrodes, and that the n-layer 4 is thereafter etched to form a channel portion.
In the above photoelectric conversion device, the capacitor C11 and the photoelectric element S11 are disposed without special isolation. This is because the photoelectric element S11 and the capacitor C11 are configured of the same layers. Such a configuration is also the feature of the photoelectric conversion device. Besides, the capacitor C11 is formed while keeping the areas of the electrodes of the photoelectric element S11 large. The reason therefor is that, when the areas of the electrodes of the photoelectric element S11 are enlarged, the sensitivity of the sensor is enhanced, leading to decrease in the quantity of exposure to X-rays as is required for the photoelectric conversion device of, for example, the X-ray imaging apparatus.
In addition, a silicon nitride (SiN) film for passivation and a phosphor layer of cesium iodide (CsI) or the like are formed at the upper part of the sensor cell. When X-rays are caused to fall on the sensor cell, they are converted by the phosphor layer CsI into light or visible radiation (indicated by arrows of broken lines), which enters the photoelectric element S11.
Next, an example of the operation of the photoelectric conversion device will be explained. Referring also to FIG. 1, the output of the charge signal converted in each photoelectric element S is stored in the storage capacitor C. The stored signal is fetched on a signal wiring line SIG when the transistor T is turned "ON" by an output signal from a shift register SR1. The fetched charge signal is inputted to a detecting integrated circuit IC when a switch M is turned "ON" by a control signal delivered from a shift register SR2.
More concretely, electric signals outputted from the sensor cells of one block are simultaneously fetched on one signal wiring line SIG, and they are collectively transferred to the detecting integrated circuit IC by the shift register SR2. Each of the electric signals transferred to the detecting integrated circuit IC is amplified into an output voltage Vout by an amplifier Amp.
The operation of the photoelectric conversion device will now be detailed with reference to a timing chart illustrated in FIG. 3. First of all, a high level voltage Hi is applied to control wiring lines g1 to g3 and s1 to s3 by the shift registers SR1 and SR2, respectively. Then, the transistors T11 to T33 and the switches M1 to M3 are turned "ON" owing to the high level outputs Hi of the shift register SR2. Thus, the electrodes D of all the photoelectric elements S11 to S33 become a ground (GND) potential. This is because the input terminal of the integrating detector Amp is designed so as to have the GND potential.
A high level voltage Hi is outputted from the refreshment control circuit RF, thereby to turn "ON" the switch SWg. Thus, the electrodes G of all the photoelectric elements S11 to S33 are brought to a plus potential by the refreshing supply voltage Vg. Then, all the photoelectric elements S11 to S33 fall into a refreshment mode and are refreshed.
Subsequently, a low level voltage Lo is outputted from the refreshment control circuit RF, thereby to turn "ON" the switch SWs. Thus, the electrodes G of all the photoelectric elements S11 to S33 are brought to a minus potential by the reading supply voltage Vs. Then, all the photoelectric elements S11 to S33 fall into a photoelectric conversion mode, and the capacitors C11 to C33 are simultaneously initialized.
Under this state, a low level voltage Lo is applied to the control wiring lines g1 to g3 and s1 to s3 by the shift registers SR1 and SR2, respectively. Then, the switches M1 to M3 for the transistors T11 to T33 are turned "OFF". Besides, the electrodes D of all the photoelectric elements S11 to S33 become open DC (direct current)-wise, but their potentials are held by the corresponding capacitors C11 to C33.
Since, however, the X-rays are not caused to fall on the sensor at this point of time, the light does not enter any of the photoelectric elements S11 to S33. In consequence, a photocurrent does not flow through any of the photoelectric elements S11 to S33. Thereafter, when the X-rays are caused to emerge in pulse-like fashion, to pass through a subject such as human body and to fall on the phosphor layer CsI, they are converted into the light, which enters the individual photoelectric elements S11 to S33. The light contains information on the internal structure of the subject such as human body. The photocurrents based on the light are stored in the individual capacitors C11 to C33 as electric charges, which are retained even after the end of the fall of the X-rays on the sensor.
Subsequently, a control pulse of high level (high level voltage Hi) is impressed on the control wiring line g1 by the shift register SR1. Besides, control pulses of high level (high level voltage Hi) are successively impressed on the control wiring lines s1 to s3 by the shift register SR2. Thus, output voltages v1 to v3 each being the voltage Vout are successively delivered through the switches M1 to M3. Likewise, the remaining light signals (or photocurrents) are successively delivered by the control signals of high level or low level (high level voltage Hi or low level voltage Lo) produced from the shift registers SR1 and SR2. In this way, the two-dimensional information items on the internal structure of the human body or the like are derived as output voltages v1 to v9.
Incidentally, a static image can be obtained by the operation stated above. On the other hand, a dynamic image can be obtained by repeating such operations.
In the photoelectric conversion device exemplified here, the electrodes G of the photoelectric elements S are connected in common to a horizontal output line. Besides, the horizontal output line is controlled to the potentials of the refreshing power source Vg and reading power source Vs through the respective switches SWg and SWs. Therefore, all the photoelectric elements S11 to S33 can be simultaneously changed over between the refreshment mode and the photoelectric conversion mode. Accordingly, the optical output can be derived by one transistor T per sensor cell without executing any complicated control.
The photoelectric conversion device has been explained above in relation to the case of transferring and outputting the signals with the construction wherein the nine sensor cells are arranged into the (3.times.3) two-dimensional arrayal, and wherein one block is constituted by the three sensor cells. However, the aspect of performance of the photoelectric conversion device is not restricted to the foregoing construction, but by way of example, (5.times.5) sensor cells numbering five per mm in each of the vertical and horizontal directions of the device may well be arranged in two dimensions into an arrayal of (2000.times.2000) sensor cells. Thus, it is possible to fabricate an X-ray detector whose size is 40 cm.times.40 cm. When an X-ray imaging apparatus or the like is constructed by combining the X-ray detector with an X-ray generator instead of an X-ray film, it can be used for a roentgenological chest examination etc.
The roentgenological chest examination with the X-ray imaging apparatus is capable of instantly projecting its result on a CRT (cathode-ray tube), unlike such an examination using the X-ray film. Further, it is capable of converting the result of detection into an output meeting a special purpose, through the digitization of the detected result and the image processing of digital data with a computer by way of example.
Besides, the converted outputs can be saved or retained in a magneto-optic disk. Accordingly, an image in the past can be instantly searched for and outputted. Moreover, the sensitivity of the X-ray imaging apparatus is higher than that of the X-ray film, and a clear image can be obtained with feeble X-rays of less influence on the human body.
Meanwhile, in manufacturing the photoelectric conversion device as stated above, some of the sensor cells fail to normally function in not a few cases. By way of example, when an amorphous silicon layer is to be deposited on a substrate, dust etc. sometimes come to lie on the substrate, but it is difficult to completely eliminate such dust etc. More concretely, when minute motes having appeared during the manufacture, trash having fallen off from the wall of a thin-film depositing equipment, etc. come to lie on the substrate, the complete elimination thereof is difficult. Accordingly, wiring lines laid on an identical plane or wiring lines laid at different levels might short-circuit.
Next, the outputs of abnormal sensor cells ascribable to the practicable short-circuits of the wiring lines of the elements will be explained in conjunction with FIGS. 4 and 5. FIG. 4 illustrates a case where the transistor T11 is in the state in which the source electrode and drain electrode thereof have short-circuited. The state results from a situation where resist patterns for forming the source electrode and the drain electrode have been connected by minute trash or the like. FIG. 5 is a timing chart of the operation of the device in such a case.
Usually, the output charges of the photoelectric element S11 are continually generated during the irradiation of the sensor with light. Besides, the charges are stored in the storage capacitor C11. However, in the illustrated case where the source electrode and the drain electrode have short-circuited, the photoelectric element S11 becomes as if it were connected with the signal wiring line SIG by turning "ON" the transistor T11. Consequently, the quantity of charges to be stored in the storage capacitor C11 becomes about 1/3 of an ordinary value. Accordingly, when the output of the photoelectric element S11 (indicated at v1 in FIG. 5) is fetched, it decreases to about 1/3 in comparison with the output of the usual operation as seen from FIG. 5.
To the contrary, when the outputs of the photoelectric elements S21 and S31 (respectively indicated at v4 and v7 in FIG. 5) are fetched, each of them has about 1/3 of the ordinary value added thereto. As a result, each of the outputs v4 and v7 increases to about 4/3 in comparison with the output of the usual operation as seen from FIG. 5.
Next, there will be explained the operation of the device in the case where the source electrode and gate electrode of the transistor T22 have short-circuited as illustrated in FIG. 4. Minute trash or the like sometimes results in the short-circuit between the source electrode and the gate electrode, as in the foregoing case of the transistor T11.
In such a case, the transistor T22 becomes as if the signal wiring line SIG and the control wiring line g2 were connected therethrough. Accordingly, the control wiring line g2 at the moment at which the outputs of the photoelectric element S12 and those S22 and S32 (respectively indicated at v2 and at v5 and v8 in FIG. 5) are fetched, is held at a potential (in general, 0 V to 5 V) in the case of turning "OFF" the transistors T. Consequently, each of the outputs of the photoelectric elements S12, S22 and S32 (respective outputs v2, v5 and v8) decreases to a much smaller value in comparison with the output of the usual operation as seen from FIG. 5.
In the presence of the defective sensor cells as stated above, the number of the erroneous outputs needs to be lessened as far as possible. It has therefore been common practice that the signal wiring lines SIG connected with the defective sensor cells are partially vaporized away to disconnect by laser irradiation.
Such a countermeasure will be explained with reference to FIG. 6. This figure illustrates the state in which the signal wiring lines SIG connected with the defective sensor cell 11 (here, the "sensor cell 11" includes the elements S11, C11 and T11, and such terminology shall apply also to the ensuing explanation) and sensor cell 22 in FIG. 4 have been partially vaporized away to disconnect by the laser irradiation.
Square parts of broken lines depicted between the signal wiring lines SIG and the respective transistors T11 and T22 of the sensor cells 11 and 22 are those parts of the signal wiring lines SIG connected with the defective sensor cells which have been vaporized away to disconnect by the laser irradiation. Incidentally, the other parts of the sensor cells 11 and 22 are the same as shown in FIG. 4 and shall be omitted from explanation.
Here, a schematic plan view of the sensor cell 11 and the individual wiring lines is illustrated in FIG. 7A. Besides, a schematic sectional view taken along dot-and-dash line 7B--7B indicated in FIG. 7A is illustrated in FIG. 7B. Further, examples of the operation of the photoelectric conversion device in the case where the defective sensor cells 11 and 22 have been subjected to the treatment or repair shown in FIGS. 7A and 7B, will be explained with reference to a timing chart illustrated in FIG. 8. By the way, in FIGS. 7A and 7B, the transistor T11 is illustrated in the state in which the source electrode and the drain electrode short-circuit. Besides, as in the foregoing, a square part of broken line in FIG. 7A is that part of the signal wiring line SIG which has been vaporized away to disconnect by the laser irradiation.
FIG. 7B illustrates the sectional structure of a portion where the signal wiring line SIG has been disconnected. As seen from the figure, not only the part of the signal wiring line SIG, but also the passivating silicon nitride layer SiN overlying the signal wiring line SIG and the gate insulator film 2, i-layer 3 and n-layer 4 underlying the signal wiring line SIG have been partially vaporized away by the laser irradiation.
Even when the signal wiring line SIG connected with the defective sensor cell 11 has been vaporized away to disconnect by the laser irradiation, the output v1 of the photoelectric element S11 does not demonstrate the value in the usual operation. The output v1 of the photoelectric element S11, however, is prevented from being superposed by leakage. Accordingly, the outputs v4 and v7 of the respective photoelectric elements S21 and S31 become as shown in FIG. 8.
Likewise, the signal wiring line SIG connected with the defective sensor cell 22 has been vaporized away to disconnect by the laser irradiation. Even when the signal wiring line SIG connected with the defective sensor cell 22 has been vaporized away to disconnect by the laser irradiation, the output v5 of the photoelectric element S22 does not demonstrate the value in the usual operation.
However, the gate line signal of the transistor T22 of the sensor cell 22 is prevented from being superposed by leakage, similarly to the effect in the case where the sensor cell 11 has been treated. Accordingly, the outputs v2 and v8 of the respective photoelectric elements S12 and S32 become as shown in FIG. 8.
Therefore, the outputs Vout shown in FIG. 5 change into those Vout shown in FIG. 8 in the above way that the signal wiring lines SIG connected with the defective sensor cells are vaporized away to disconnect by the laser irradiation. More specifically, the outputs Vout shown in FIG. 8 have been improved so as to be normal except the outputs v1 and v5 of the respective defective sensor cells 11 and 22.
Nevertheless, diminishing the X-rays influential on the human body, even slightly, is required of the photoelectric conversion device which has the photoelectric elements arrayed in two dimensions for the actual-size reading in, for example, the X-ray imaging apparatus. It is also required to obtain data of higher precision.
In order to meet the requirements, an expedient in which the area of each photoelectric element is enlarged has been generally adopted. Since, however, semiconductor patterns have been made highly dense, signal wiring lines connected with sensor cells have been reduced in size. It is accordingly difficult that, as stated above, the signal wiring lines connected with the defective sensor cells are partially vaporized away to disconnect by the laser irradiation.
More specifically, in case of manufacturing the photoelectric conversion device of high S/N (signal-to-noise) ratio or high resolution, the signal wiring lines connected with the sensor cells are partially used for the photoelectric elements, and yet, they are reduced in size or removed. Therefore, it is sometimes impossible that the signal wiring lines connected with the defective sensor cells are partially vaporized away to disconnect by the laser irradiation. Accordingly, it is sometimes impossible that the outputs of the sensor cells except the defective ones are brought to the usual values.